1. Field of the Invention
The invention relates in general to a receiver, and more particularly to a receiver that pre-filters according to a channel estimation result.
2. Description of the Related Art
In modern communication technologies, a transmitting end and a receiving end adopt a communication protocol or standard understood by both parties to promote the communication between the two parties. Signals transmitted by the transmitting end pass through a transmission channel and are received by the receiving end. As the signals may be mixed with various interferences and noises in the transmission channel, the signals may attenuate during the transmission process. Thus, the receiving end frequently utilizes a known pilot signal in the signals to perform channel estimation. Having estimated the effect that the channel poses on the signals, the received signals are equalized to restore the received signals into original signals transmitted from the transmitting end.
FIG. 1 shows a schematic diagram of a model of channel propagation in the prior art. A signal transmitted from a transmitting end 110 is represented by x(n), and a signal received at a receiving end 130 is represented by y(n). The signal x(n) is distorted and contaminated when passing through a channel 120.
Under certain circumstances, the channel 120 may result in a more complex multipath effect. The so-called multipath effect refers to an effect that the signal x(n) arrives at the receiving end 130 at different time points via multiple paths due to different lengths of the paths. Not only the signal strength of the signal x(n) transmitted via the paths is attenuated, but also the signal arriving at different time points may pose interferences on subsequent signals. In FIG. 1, the influence that the multipath effect poses on the signal is represented by h(n).
In addition to the signal distortion caused by the multipath effect h(n), a channel 120 that does not suffer from particular interferences further receives random interferences. Such random interferences are commonly referred to as the additive white Gaussian noise (AWGN), which is represented by w(n) in FIG. 1.
Having undergone the distortion and contamination of the multipath effect h(n) and the AWGN w(n), the signal received at the receiving end 130 passes through a channel estimation module 132. By utilizing channel impulse response (CIR) estimation, the channel estimation module 132 sends an estimated message to an equalizer 134 to compensate the distortion and contamination that the channel 120 causes on the signal. Thus, a compensated signal {circumflex over (x)}(n) outputted by the equalizer 134 is similar to the signal x(n) transmitted from the transmitting end 110.
Although the above conventional channel estimation module 132 treats the multipath effect, for a signal that has the same signal strength and arrives via numerous different paths, the compensation provided by the equalizer 134 still requires a lengthy period for iterated processes on the signal to converge to a usable level.
FIG. 2A shows a schematic diagram of a multipath effect. In FIG. 2A, a multipath effect h(n) of a signal that is transmitted from the transmitting end 110 and passes through three paths is depicted. After passing through the three paths, due to differences in the lengths of the paths, the signal arrives the receiving end 130 at time points that are spaced by an equal interval. Further, the energy of the signal is evenly propagated along the three paths, meaning that the amounts of energy allotted to the paths are substantially equal. In other words, none of the three paths is considered as a primary path having the highest amount of energy or a secondary path having a lower amount of energy.
FIG. 2B shows a diagram of actual measurements of the multipath effect in FIG. 2A. In FIG. 2B, three rising energy peak values represent the transmission time of the signal in the three different paths.
In a conventional design of the equalizer 134, e.g., a decision feedback equalizer (DFE), a feedback signal is processed in iteration for a received signal. When the multipath effect h(n) shown in FIG. 2A and FIG. 2B is received, the primary path having the highest amount of energy cannot be determined, and so the equalizer 134 requires a quite long time to adjust parameters to achieve convergence.
FIG. 3 shows a diagram of a signal-to-noise ratio (SNR) of the equalizer 134 versus the time under the multipath effect in FIG. 2A and FIG. 2B. Starting from the signal of the first path, the equalizer begins iterated calculations. However, since which of the paths is the primary path having the highest amount of energy cannot be determined, the equalizer needs to spend more than one second to increase the SNR of the output signal to a level that can be applied in subsequent processing.
For high-speed modern communication systems, it is unacceptable that the equalization requires such long convergence time under a multipath effect. Therefore, there is a need for an apparatus and method that overcomes the multipath effect and enables equalizer 134 to timely output a high SNR to further allow the receiving end 130 to quickly receive the signal.